(i) Field of the Invention
The present invention relates to novel microporous zirconium silicate compositions that are formulated to remove toxins, e.g., potassium ions or ammonium ions, from the gastrointestinal tract at an elevated rate without causing undesirable side effects. The preferred formulations are designed to avoid potential entry of particles into the bloodstream and potential increase in pH of urine in patients. These compositions are particularly useful in the therapeutic treatment of hyperkalemia. Also disclosed are microporous zirconium silicate compositions having enhanced purity and potassium exchange capacity (KEC), and methods and apparatus for making such microporous zirconium silicate compositions.
(ii) Description of the Related Art
Acute hyperkalemia is a serious life threatening condition resulting from elevated serum potassium levels. Potassium is a ubiquitous ion, involved in numerous processes in the human body. It is the most abundant intracellular cation and is critically important for numerous physiological processes, including maintenance of cellular membrane potential, homeostasis of cell volume, and transmission of action potentials. Its main dietary sources are vegetables (tomatoes and potatoes), fruit (oranges, bananas) and meat. The normal potassium levels in plasma are between 3.5-5.0 mmol/l with the kidney being the main regulator of potassium levels. The renal elimination of potassium is passive (through the glomeruli) with active reabsorption in the proximal tubule and the ascending limb of the loop of Henle. There is active excretion of potassium in the distal tubules and the collecting duct, both of which processes are controlled by aldosterone.
Increased extracellular potassium levels result in depolarization of the membrane potential of cells. This depolarization opens some voltage-gated sodium channels, but not enough to generate an action potential. After a short period of time, the open sodium channels inactivate and become refractory, increasing the threshold to generate an action potential. This leads to impairment of the neuromuscular-, cardiac- and gastrointestinal organ systems, and this impairment is responsible for the symptoms seen with hyperkalemia. Of greatest concern is the effect on the cardiac system, where impairment of cardiac conduction can lead to fatal cardiac arrhythmias such as asystole or ventricular fibrillation. Because of the potential for fatal cardiac arrhythmias, hyperkalemia represents an acute metabolic emergency that must be immediately corrected.
Hyperkalemia may develop when there is excessive production of serum potassium (oral intake, tissue breakdown). Ineffective elimination, which is the most common cause of hyperkalemia, can be hormonal (as in aldosterone deficiency), pharmacologic (treatment with ACE-inhibitors or angiotensin-receptor blockers) or, more commonly, due to reduced kidney function or advanced cardiac failure. The most common cause of hyperkalemia is renal insufficiency, and there is a close correlation between degree of kidney failure and serum potassium (S-K) levels. In addition, a number of different commonly used drugs cause hyperkalemia, such as ACE-inhibitors, angiotensin receptor blockers, potassium-sparing diuretics (e.g. amiloride, spironolactone), NSAIDs (such as ibuprofen, naproxen, celecoxib), heparin and certain cytotoxic and/or antibiotic drugs (such as cyclosporin and trimethoprim). Finally, beta-receptor blocking agents, digoxin or succinylcholine are other well-known causes of hyperkalemia. In addition, advanced degrees of congestive heart disease, massive injuries, burns or intravascular hemolysis cause hyperkalemia, as can metabolic acidosis, most often as part of diabetic ketoacidosis.
Symptoms of hyperkalemia are somewhat non-specific and generally include malaise, palpitations and muscle weakness or signs of cardiac arrhythmias, such as palpitations, brady-tachycardia or dizziness/fainting. Often, however, the hyperkalemia is detected during routine screening blood tests for a medical disorder or after severe complications have developed, such as cardiac arrhythmias or sudden death. Diagnosis is obviously established by S-K measurements.
Treatment depends on the S-K levels. In milder cases (S-K between 5-6.5 mmol/l), acute treatment with a potassium binding resin (Kayexalate®), combined with dietary advice (low potassium diet) and possibly modification of drug treatment (if treated with drugs causing hyperkalemia) is the standard of care; if S-K is above 6.5 mmol/l or if arrhythmias are present, emergency lowering of potassium and close monitoring in a hospital setting is mandated. The following treatments are typically used:                Kayexalate®, a resin that binds potassium in the intestine and hence increases fecal excretion, thereby reducing S-K levels. However, as Kayexalate® has been shown to cause intestinal obstruction and potential rupture. Further, diarrhea needs to be simultaneously induced with treatment. These factors have reduced the palatability of treatment with Kayexalate®.        Insulin IV (+ glucose to prevent hypoglycemia), which shifts potassium into the cells and away from the blood.        Calcium supplementation. Calcium does not lower S-K, but it decreases myocardial excitability and hence stabilizes the myocardium, reducing the risk for cardiac arrhythmias.        Bicarbonate. The bicarbonate ion will stimulate an exchange of K+ for Na+, thus leading to stimulation of the sodium-potassium ATPase.        Dialysis (in severe cases).        
The only pharmacologic modality that actually increases elimination of potassium from the body is Kayexalate®; however, due to the need to induce diarrhea, Kayexalate® cannot be administered on a chronic basis, and even in the acute setting, the need to induce diarrhea, combined with only marginal efficacy and a foul smell and taste, reduces its usefulness.
The use of zirconium silicate or titanium silicate microporous ion exchangers to remove toxic cations and anions from blood or dialysate is described in U.S. Pat. Nos. 6,579,460, 6,099,737, and 6,332,985, each of which is incorporated herein in their entirety. Additional examples of microporous ion exchangers are found in U.S. Pat. Nos. 6,814,871, 5,891,417, and 5,888,472, each of which is incorporated herein in their entirety.
The inventors have found that known zirconium silicate compositions may exhibit undesirable effects when utilized in vivo for the removal of potassium in the treatment of hyperkalemia. Specifically, the administration of zirconium silicate molecular sieve compositions has been associated with an incidence of mixed leukocyte inflammation, minimal acute urinary bladder inflammation and the observation of unidentified crystals in the renal pelvis and urine in animal studies, as well as an increase in urine pH. Further, known zirconium silicate compositions have had issues with crystalline impurities and undesirably low cation exchange capacity.
The inventors have discovered novel zirconium silicate molecular sieves to address the problem associated with existing hyperkalemia treatments, and novel methods of treatment for hyperkalemia utilizing these novel compositions.